Sodium sulfite recovery process



Oct. 24, 1961 G. G. DE HAAS soDIUM SULFITE RECOVERY PROCESS Filed May 5, 1958 BY @err/t G. D @HOUS A Tram/5y United States Filed May 5, 1958, Ser. No. 733,177

1 Claims. (Cl. 23-131) This invention relates to a process for the recovery of the sodium and sulfur values from the spent liquors of kraft, neutral sodium sulte and, in particular, acid sodium suliite wood pulping processes in the form of sodium suliite and sodium bisulte. More specifically, the new process relates to a cyclic recovery procedure in which the combined sulfur of the said spent liquors is separated from the sodium ions in the form of sulfur gases and recombined to produce the sodium sulte for return to the pulping process.

It is well known to concentrate the waste liquors from wood pulping processes such as the kraft, neutra sodium sulte and acid sodium sulite pu-lping processes and burn o the organic matter in a smelting furnace to produce an inorganic ash or smelt consisting pfrnn-arily of sodium sulte and sodium carbonate. This smelt residue, in accordance with conventional procedures, is dissolved to produce what is known as green liquor. The conversion of this green liquor to a product which can be returned successfully to the sulte pulping process has presented diiculties which make such a conversion an uneconomic consideration. It therefore has been common practice to dispose of the waste liquor by dumping it into streams or other bodies of water, thus producing pollution problents.

One of lthe principal difficulties encountered in the conversion of the green liquor to a solution containing sodium, sulte, and possibly carbonate-ions results from the fact that if sulfide and suliite ions are present in the same solution -a considerable amount of sodium thiosulfate is formed, particularly at lower pH conditions. Sodium thiosulfate is undesirable in acid cooks. It also materially reduces the quantity of sulfur that can be recovered to form sodium sul-te, since each molecule of sodium thiosulfate contains twice as much sulfur per Na moiety `as is contained in sodium sulte. This greatly hampers the e'icient recovery of the sulfur compounds and results in an accumulation of useless chemicals recirculating through the recovery system. Recovery processes have been suggested which are directed to minimizing or preventing the formation of sodium thiosulfate. Such processes have in common the expedient of subjecting the green liquor containing sodium sulde to the action of carbon dioxide, thereby to generate hydrogen sulde and -at the same time form a mixture of sodium carbonate and bicarbonate. However, it has been found diicult to convert the Na2S practically completely to sodium carbonateebicarbonate and to `convert the H formed to SO2 using no or little additional heat and without a tiedn with the recovery furnace, while at the same time eliminating the danger of explosions and the escape of toxic H28. Other difficulties that are often encountered are scaling of heat exchanger surfaces that are in contact with green liquor or the liquors produced from the green liquor, the precipitation of solids produced during the conversions, and a high stearn consumption.

I have now devised a procedure which overcomes these diiculties and provides `a recovery process for the sodium and sulfur values of green liquor which may be operated continuously for the effective conversion of the sodium compounds contained in the green liquor to sultes capable of being re-used in the wood pulping digesters. This procedure provides a system which involves recirculation arent 3,005,686 nPatented Oct. 24, 17961 ICC of liquor and gas streams that can be controlled auto matically and require a minimum of supervision.

In accordance with the improved process of this invention the green liquor is reacted in a packed tower at a slight vacuum with a gas containing principally nitrogen and carbon dioxide. While the gases llow up the tower countercurrently to the liquor they convert the sodium sulde contained in the green liquor to sodium carbonate and strip off t-he hydrogen sulfide that has been formed. The hydrogen sulde concentration of the gas may vary considerably. It is low at the bottom of the tower, increases while passing up the tower, and finally decreases as it approaches the top of the tower. If the gas ow is reduced sufliciently, the concentration of the hydrogen sulfide as the gas leaves the system can be reduced to an insignificant value. The carbon dioxide concentration will also be removed. This is accomplished by carrying out the carbonation of the sodium suliide content of the green liquor in two stages. In the stage just prior to exit of the gases from the system, hereinafter referred to as the absorption stage or the irst stage carbonation, the -hydrogen sullide becomes absorbed as it advances to the top of the tower against the ow of the green liquor, and the carbon dioxidelpresent in that stage reacts with the sodium sulide in the lower regions of the `same tower. In order to convert the sodium suliide to sodium sulte, it is necessary to introduce oxygen to the system. Economically this is introduced in the form of `air so that a corresponding quantity of nitrogen is also introduced into the system, and unless it is purged it will tend to accumulate. In accordance with the operation of the absorption stage in the aforesaid tower, a substantial part of the nitrogen added with the air can be relieved from the system without removing any significant amount of hydrogen sulfide by reason of the absorption of practically all of the HZS and CO2 from the controlled ow of gas containing these compounds and the nitrogen.

It was found that practically all of the sulfide content of the green liquor may be removed by contacting it with a large quantity of gas containing a Ahigh percentage of CO2 passing countercurrently through a carbonation tower, while at the same time producing a gas which contains a suitable concentration of hydrogen sulde which may be oxidized catalytically to the sulfur dioxide or elemental sulfur without the necessity of adding heat to raise the temperature of the gases, and with practically no sulfur trioxide formation. Highhydrogen sulde concentra-v tions tend to produce explosive mixtures. It is an advantage of this process that the hydrogen sulfide concentration of the gas piped to Athe catalyst may be maintained below the limits of inflammability, but still sufficiently high to provide suliicient heat of conversion to raise the temperature of the gases .to the desired level for conversion of the hydrogen suliide to the sulfur dioxide or elemental sulfur. The maximum concentration of the HRS in the gas is 6%Y based on the total volume of the gas minus the oxygen content. The minimum concentration depends on the-amount of hot gases leaving the catalyst, that is returned together with the fresh unreacted gas entering the catalyst system and the availability of a gas heat exchanger. However, generally the minimum concentration of H2S will be kept in excess of 1%. 'Ihe temperature of the catalyst and gases should be at least 200 C., but shouldnot exceed 400 C.

This process' is applicable to the treatment of green liquor containing either a high or a low percentage of sodium sulfide. The in uence of a decrease of the sodium sulfide concentration on the concentration of the hydrogen sulfide available from the carbonation stage is counter acted by an increase in the CO2 concentration produced in the later stage suliitationV and recirculated to the primary carbonation stage to strip out the H28. This increase in the C02 concentration is made possible by the higher concentration of sodium carbonate present in a green liquor of low sullidity. The `gas volume needed to strip olf a certainA amount of H2S can be reduced if'the CO2 concentration of the stripping gas' increases. Gas from the catalytic oxidation of H2S, after scrubbing to eliminate sulfur containing gases, is recirculated. As the gas, that is'recirculating through the system, originates from air and CO2, developed from thecarbonate-bicarbonate solution, it is relatively pure, and the result is a minimum ot fouling of the catalyst. Another advantage of the system is that the CO2 concentration builds up to a higher concentration than is found, for example, in line gases. resulting in a more complete conversion of the Na2S in a smaller carbonation tower.

As the description of the illustrative modification of the invention progresses, it will become apparent that certain controls are possible and necessary to the continuous automatic operation' of this process. Thus, within relatively Wide limits oi green liquor vflow and concentrations ofV the sodium sulfide and sodium carbonate, the following variables may be utilized effectively as indicators by which to control the( efficiency of the process:

gas leaving the catalyst;

suliite cooking liquor Vfrom a green liquor containing lowV or high percentages of sodium sultide.

Y Referring nowV to the gure of the drawing, tower 3-1 is an absorption tower, and tower 36 is a carbonation tower. Both towers contain a special type of .tower packingv consisting of layers of coils made up of strips of sheet metal. A corrugated strip and al at `strip are rolled up to a coil that just lits into the tower. Operational diiculties encountered because of accumulations in the towers of dirt and sludge originating from the green liquor are eliminated by this type of packing.

The absorption of CO2 and H2S in green liquor and the coversion of the sodium suliide 'content to sodium carbonate and H2S are preferably carried out in two separate towers. As illustrated in the flow sheet, the ligure of the drawing, .the conversions are eiected by novel combinations of re-cycling operations. The green' liquor 30 enters the top of absorption tower 31. It descends countercurrently to a part 32 of the gas product 18 containing CO2 and H2Sin addition to nitrogen and an insigniicant'amount `of oxygen. The gas leaving the towery at 35 and through fan 28 consists primarily of nitrogen. Fan 28 puts the system under a slight vacuum, thus pre'- venting any contamination of the area around the conversion plant by toxic H25. The gas relieved from fan 28 can be piped to the regular recovery furnace stack or used as asource of nitrogen. Traces of CO2 and H28 can be removed in a scrubber operatedrwith make-up caustic. `Some of the sodium sulide ofj the green liquor is converted to sodium carbonate and sodium bicarbonate in this tower. The H2S liberated by this conversion is absorbed in the liquor as NaHS. Liquor in the sump of the absorption tower 31 is then pumped to the .top of the carbonation tower 36 by pump 37 through line 38. Suthcient gas containing CO2 is passed through the carbonation tower 36 countercurrently to the liquor to convert the NaHS and Na2S in the liquor to Na2CO3 and NaHCO3, and to strip out the H2S absorbed in the liquor and formed in the Ycarbonation tower. The major portionof gas 18 leaving'the carbonation tower and containing the H2S is piped through a gas washer 29 and conduit 34 to the catalyst chamber 39, and suiiicient air, or oxygen, is mixed with it at 40 for the catalytic conversion of all the H25 to SO2, or sulfur and water vapor. The temperature of the gas mixture entering the catalyst chamber is raised by recirculating, by means of fan 27, a major part of the hot gases leaving the catalyst through conduit 41. It desired, a gas heat exchanger for the gases of 34 and 42 may be used. Such a heat exchanger should be placed in such a position that any liquied elemental sulfur runs with the gas into cooler 43 and separator box 44.

The cooler 43 cools the gases down to 1Z0-150 C. The elemental sulfur is piped to the sulfur burner through 45. A gas washer 46 may be used to remove any carryover of elemental sulfur, and any residual H28 and SO2 will react in the presence of water to form elemental sulfur in the gas washer 46 which is operated with spent liquor or water. The liquor effluent from it 72 may be united with the spent liquor piped to the recovery furnace.

When the green liquor is of low suliidity it is preferable to convert the H2S to SO2 since more air can be used without unnecessarily diluting the CO2. The available CO2 increases as the suldity decreases. Conversely, with a high suldity green liquor the available CO2 is less and conversion of the H28 to S is preferred in order to keep the dilution of the CO2 to a minimum. Stoichiometric quantities or a slight excess of oxygen are introduced with the air when it is desired to oxidize to elemental S, and a controlled excess when it is desired that the reaction go to SO2. Too great an excess in the latter case will promote the formation of S03, particularly at high temperatures.

The catalyst may be any conventional catalyst suitable for the conversion of H2S to SO2 or S, but preferably is precipitated platinum or nickel carbonate on a carrier. The catalyst may be prepared as described in British Patent No. 675,349. Although the same catalyst may be used for either reaction, the nickel catalyst gives the highest eiliciency for conversion to SO2. Forrconversion to elemental S it has been found that an aluminum base catalyst is best. For the preparation of thevaluminum base catalyst two or more ofthe following compounds, one being an aluminum compound, are precipitated on a carrier from aqueous solution: aluminum hydroxide, aluminum sulfate, sodium silico liuoride, ammonium chloride, and sodium silicate. The catalyst is preheated in the catalyst chamber 39' to a temperature of from 200-350 C., Vand is maintained at a temperature not in excess of 400 C. Some of the moisture carried -by the entering gas may be removed in the gas Washer 29 by condensation before the gas Vpasses through the catalyst chamber 39. A quantity of nickel salt containing one pound of nickel was sufcientto produce enough catalyst to convert to SO2 more than 98% of the H28 contained in 45 cu. ft. of gas per minute at a temperature of 240 C. The gas contained 30% CO2, 15% H2O and 2.5% H28 in addition to nitrogen and oxygen. At higher inlet temperatures considerably higher gas ilow rates could be handled. In the case where the H2S is converted to elemental sulfur, a portion of the gas @may be used in the absorption tower `31 instead of the gas piped from line 18.

The liquor in the sump of the carbonation tower 36 containing sodium carbonate and sodium bicarbonate is pumped by means of pump 26 directly to a mixing tank 50. Sodium bisuliite solution from a sultation tower 52 is piped through line 53 to join line 55 entering mixing tank 50. The liquor and gas temperatures in towers 31 and 36 and tank 50 are approximately lthe same (6G-90 C.), and therefore little or no steam is used up in the mixing tank 50. Some steam is added to the mixing tank only if the temperature of the bisulite liquor 53 from the sultation tower is lower than that of towers 31 and 36. Under normal conditions the liquor leaving the carbonation tower contains only traces of or no Na2S and NaHS. However, it was found that if for some reason a significant amount of suliide does appear in this liquor the residue sulide can be stripped off by passing steam and the CO2 developed in tank 5 0 first countercurrently to the liquor 55 through a relatively small tower. IIn this case the gas 60 is not added to tank 50, but to the upper part of the small tower.

Excess gas 68 is piped to the bottom of the sulfitation tower. Instead of the mixing tank 50 a small tower may be used. The sodium carbonate-bicarbonate solution 55 and the bisuliite solution 53 are sprayed into the top. The CO2 developed by the reaction is stripped from the liquor by the same steam introduced at the bottom of this small tower. The gas 60 fromthe catalyst system may be piped to the bottom of the tower instead of steam, or it is piped directly to line 17.

The liquor in the mixing tank 50 containing essentially sodium, sulfite, and carbonate ions is pumped through conduit 23 to the top of the 'bisuliite or suliitation tower 52 for countercurrent passage with gases 57 from a sulfur burner and the HZS catalytic converter if SO2 is produced in the` latter. The SO2 reacts with the sodium suliite to produce sodium bisulite in the sulfitation tower. The sodium ibisultite solution is then returned to the mixing tank 50 through line S3.

The temperature of the liquor in the carbonation tower generally should not drop below 65 C. in order to prevent the crystallization of sodium bicarbonate. It is preferable that the liquors in the system be maintained at a temperature of about 70 C. by introduction of the green liquor at that temperature and addition of such amounts of steam as may be made necessary fby heat losses.

In the following examples which are intended to be illustrative, and not limitative, of the invention, the treated liquor is a smelt liquor derived from a kraft and a neutral sulfite pulping process to which sodium sulde has been added in order to make the treated liquor representative of an acid sulfite green liquor. This was done since it was a particular objective of the development of this process to produce an acid suliite cooking liquor from the waste liquors. However, it is to be understood that the process can be used for the production of a neutral sulte cooking liquor from the waste liquors of a neutral sulte process. Since the cooking liquor for a neutral sulte pulping process involves dilerent quantities of chemicals per unit of neutral suliite pulp, the ow rate of the smelt liquor to the recovery system would be adjusted to provide the chemicals necessary to produce the amount of cooking liquor required per unit of time.

In each of the examples the equipment shown in the iow diagram of the gure of the drawing was employed in a pilot plant system. The absorption tower 31, the carbonation tower 36, and sultation tower 52 were packed with the special type of packing described above. The smelt liquor from a kraft and neutral suliite mill was piped to a mixing tank where the sodium sulde necessary to increase the suldity to that of a typical acid sulte green liquor was added.

EXAMPLE 1 Acid sulte green liquor from the above-mentioned mixing tank was delivered at a temperature of 70 to 80 C. to the -top of the absorption tower 31. The said green liquor Was delivered at such a rate as to provide the chemicals necessary to produce 148 gram moles per minute of combined (Na2SO3 calculated as SO2) or the 30,000 lbs. per day necessary to provide a cooking liquor for an acid sulte mill producing 100 tons of pulp per day. At the dilution used the ow rate was Iabout 20 gallons per minute. This provided the following chemicals at the beginning of the absorption stage:

S- 78.3 moles/min.=6.1 kg./min.Na2S. (1) {C03-. 9.7 moles/min.=7.4 kg./min.Na2CO3. S2032- 2.11 moles/m1.u.=0. 33 lig/mm. NazSiOs.

This green liquor descended in the absorption tower against a countercurrent flow of a part 32 of the gas 0.15 nimes/minfo. 2% 1.0 moles/ruin.-- 1.5% 68 eu. ft/min. 66.9 moles/min.-98. 3%

a) {Cm- 33:33:

Liquor (3) was pumped `by means of pump 37 through line 38 tothe top of carbonation tower 36 where it descended countercurrently to a tlow of gas 17 from the mixing tank 50. 'I'his gas consisting of:

Nn(+0z)--- 1,620 moles/min.-52. 5% 5) CO2 1, 470 mo1es/min.-47. 6% 3,091 cu.ft./rn1n.

1.28 moles/min.

. reactedwith liquor (3) according to following equations:

S' (6) C03-. 219. 5 moles/min.=7. 93 kg. NagCOa; 12.15 kg. NaHC Og.

S203"- 2. 11 mOleS/mIL=0. 34 kg. NazSzOa.

and a gas 18 leaving the Atop of they tower consisting of:

HzS 81. 1 moles/ming 2.6% l A part of gas (7) was returned to `the absorption tower 31 through line 32 as -gas (2). The remainder of gas (7) was mixed with air and the mixture conducted to catalyst chamber 39 which had been preheated to about 240 C. The mixed gas consisted of:

N,(+02) 1, 553 mummia-49. 3% (8) {COz 1, 322 moles/min.-42.0%}3,148 cu. t./min.

H 77. 7 moles/min.- 2. 5%

195. 2 moles/min.- e. 2%

i The H28 of gas (8) was converted to elemental sulfur which was piped to the sulfur burner and a gas product consisting of:

N2(+Oz)--- 1, 709.3 mummia-56.407 ico, 1,322 moles/mmrsjaoso ou. amm.

which was piped to the mixing tank 50 (2873 cu. ft./min.) and the excess (157 cu. ft./min.) vented to atmosphere through the sultation tower 52.

The liquor (6)` -from the sump of carbonation tower was pumped to the top'of mixing tank 50 where it was mixed with liquor (11) from the sultation tower 52. The following reactions took place in this step:

producing -a product -liquor and a product gas (S). The latter was recirculated via 17 to the bottom of the carbonation tower 36, as indicated above. The liquor product lwas divided, one part being withdrawn as product, and the other part delivered to the top of suliitation tower 52. (The ow of liquor used to make bisulte solution depends on the other facilities available in the mill. In

the acid suliite mill, for example, a lower flow will be sufficient because' a liquor with a relatively high ooncentration of free SO2 will be available, while in a neutral sulite mill higher flows will be used, especially if the sultation towerpis operated at temperatures up to andasse 70 C.) These A'portions of product liquor each Sion-` tamed: Y

SC3- 152.1 moles/min.=18.23 kg. NaaSOa; 0.77 kg. NaBSOa. (9) C03- Traces.

S203'- 2.71 m0leS/111i11.= 0.43 kg. NazSzOs.

' The liquor (9) descends through the sultation `tower countercurrently to the ow of gas from the sulfur burner which latter has the composition:

in such a way that va significant amount of SO2 appears in the recovery furnace due gases, `the liquor from the mixing tank l) can be passed first through auxiliary towers countercurrent to the ue gases and then used in the sulfitation tower 5.2.

EXAMPLE 2 I. Absorption and carbonation section In this example the same procedure and equipment as used in Example 1 were used, except that the HZS was catalytically oxidized to SO2 instead of sulfur in the catalyst chamber. The quantities `are on a per minute basis. A portion of the product gas from the catalyst was delivered to the sultation towervia 68, and the remainder 47 was delivered to the mixing tank via 60 for reaction with the NaHCO-a .content of the mixed liquor as follows:

zivancoa-tsoz1112.503+H20+2C02 The assay conditions obtaining in the Various stages of the process were as followsz Green liquor entering the top of the absorption tower:

57.-..- 52. 35 mo1es=.4.08 kg. NazS. Y (1) C03- 96.4 moles=10.22kg. NazCOs. 5203-.. 1.28 m0lBS= 0.20 kg. NazSzOs.

Gas entering the bottom of the absorption tower- This is a part of the gas leaving the top of theVcar-bonation tower: Y

HgS 5.43 moles- 2.9% (2) COL.. 45.33 moles-23.9% 189 cu.t./min.

NiH-.O Y.- 138.3 moles-73.2%

Liquor leaving the bottom of the absorption tower- This liquor is piped to the top of the carbonation tower:

s 57.8 m01es= 3.23 kg. Nans. l (3) C03-. 140.8 mo1es=10.49 kg. Na2003; 3.52 kg. NaHCOa.

S203 l. 28 m0leS= 0. 20 kg. N31S203.

Gas leaving the top of the absorption tower-This gas is released to the atmosphere:

HzS 0.08 moles- 0.06% CO2 0.93 moles-0.60% 139 cuit/min. Nui-02)-. 138.3 moles-99.30%

Gas entering the bottomrof the carbonation tower--V .',lhisrgas comesY from the mixing-bisulii-te section:

Gas leaving the top of the carbonation tower--Part of As was mentioned above, in case the mill is operated esV this gas is piped to the absorption tower and part to the catalyst section: i

Hts 5s mo1es- 2.9% (7) {CO2 460 moles-23. 9%}1, 918 cuit/min.

Nrw-02)-- 1,402 moles-73.2%

II. Catalyst section Gas entering the catalyst system-Thiais a part ofthe gas leaving the top of the carbonation tower:

moles-73.2%

Liquor from carbonation tower delivered to mixing tank 50:

(See above (6)) Liquor from bisulfite tower delivered to mixing-tank 50:

(lo) {SC3- 209.8 moles=10A0 kg. NazSOB; 13.23 kg. NaHSOz.

S2011-- 3.98 moles=0.63 kg, Na2S2O3. Y

Gas from the catalyst section (part of 9):

NMi-01)..; 1, 402 moles-77. 2% (ll) CO2 369.5 moles-20. 3% 1,817 cu.ft./ml1.

2 45. 2 moles- 2. 5%

Liquor leaving tank S0 as produced liquor:

503-..- 125.6 moles=15.83 kg. NazSOa.

(12) COS- 25. 5 moles= 1.63 kgNmCOa; 0.8 5 kg. NaHCOs.

S2031.- 3.98 moles= 0.63 kg. NaSzSzOa.

Liquor leaving tank 50 to be recirculated through b.i. sulte tower 52: Y

sol- 125.6 m01es=15.sskg.1\ra2so3. (13) {C03" 25. 5 moles= 1. 63 kg. NazGOz; 0.85 kg. NaHCOa.

5203-.-- 3.98 moles= 0. 63 kg. NazSzOa. Gas from tank 50 to .be recirculated to bottom of the carbonation tower via 17:

(See above (5)) Gas entering the bottom of the bisulte tower:

A. From Catalyst Section via 68 (Part of 9) N201-O1) 169. 6 moles-77. 2%

(14) {002 44. 5 moles-20.3%}219 cu. ftjmin.

1 5.4 moles- 2.5%

B. From the Sul-fur Burner via 57 (15) ,2H-02)..- 393.5 moles-83.3% .SC2 78.6 moles*16.7%

Liquor leaving the bottom of the bisuliite tower 52: This liquor is returned to the top of the mixing tank 50:

(See above (10)) 72 eu, ft./1nin.

Gas leaving the top of the :bisulfite tower- This gas is piped to the ue gas duct:

NZH-01)-.. 563.1 moles-92.8% (16) CO2 43.8 ruolcs- 7. 2% 607 cu. ftjmin.

SO2 Traces.

EXAMPLE 3 't Y I. Absorption and carbonation section Green liquor enter-ing the top of the absorption tower at 70-80 C. provided the following quantities of the identified chemicals. (All quantities are on a per minute basis).

36 moles= 2. 81 kg. NaiS.

S- (l) C03- 114 moles=12-08 kg. NazCOa. Sgm- 1.4 moles= 0. 22 kg. Na2S2O3.

Gas havingthe` following composition, and being part of the gas leaving the top of carbonation tower 36, was

9` delivered to the bottom of absorption tower 31 to iiow countercurrently to the green liquor:

HQS 3. 63 moles- 2. 4% (2) {002 72.4 mo1es48. 8%}148 ou. ft./min.

NZH-01).-- 72. 5 moles-48. 8%

The H2S and CO2 content of the gas were absorbed in the green liquor and reacted with the Na2S to produce NaHS, Na2CO21 and NaHCO3.

The liquor leaving the bottom of the absorption tower had the following composition:

S- 39. 58 moles=2. 22 kg. NaI-IS. (3) {C03- 185 rnoles=8. 00 kg. NazCOa; 9. 19 kg. NaHCOa.

:Os-* 1.4 mleS=O. 22 kg. NazSzOg.

The gas leaving the top of absorption tower 31 had the following composition: 4) {Tdro i5 moies- 25% 74o f The liquor (3) was pumped via 3.7 to the top of carbonation tower 36 and allowed to fall freely through the packing against the ascending ilow of gas entering at the bottom via conduit 17 having the following composition:

B2S 0.4 moles 5) CO2 825. 2 moles-51.6%}1,598 cu. ft./n1in.

The CO2 reacted with the NaHS and Na2CO3 to produce H28 and NaHCO3. The liquor leaving the bottom of the carbonation tower had the following composition:

S- 1.4 moles=0.08 kg. NaHS. (6) C03- 240 moles=.34 kg. NarCOs; 15.13 lg. NaHOO:. S2O3 1.4 mo1es=0.22 kg. NazSaOg. and the gas leaving the top of the carbonation tower analyzed as [follows:

B2S 38.58 moles- 2.4% (7) Co2 770.2 mo1es-1s.s%}1,58o cart/min.

N,(+Og). 771.6 moles-48.8%

II. Catalytic section A major part of said gas (7) having the following compositlon:

HQS 34.95 moles- 2.4% (8) C02 697.8 moles-48.8% 1,432 aufn/min.

Air added.- 262 moles was delivered to the catalytic chamber 39 via conduit 34. The remaining part (2) was delivered to the absorption tower Sil as stated above.

The catalytic chamber was preheated to reaction temperature of about 300 C., and the temperature was maintained under 400 C. by recycling a controlled portion of the hot reaction gases. The H2S of the gas (8) was completely converted to SO2, the gas leaving the catalytic chamber consisting of:

SO2 34. 95 moles- 2.1% (9) CO1 697.8 moles-42. 5% 1,641 cu.ft./mln.

Nz(+02)- 908. 6 moles-55. 4%

III. Mixing section The liquor product (6) of the carbonation reaction was delivered from the bottom of the carbonation tower 36 to the mixing tank 50. To this was added the liquor product of the sultation or lbisullte tower 5-2 having the following composition:

so 276.35 01s=2.e k .N gsomaek .N nso. (lo) {Stoas- 2.0 iiliois=o-46 kg. Nzsnia. g a 3 A part of the gas product (9) of the catalytic conversion having the .following composition:

SO2 29. 65 moles- 2.1% CO2 592 moles-42. 5% NZH-02)---. 771. 6 mons-55.4% was also delivered to the mixing chamber where the following reactions took place:

'(11) }l,393 cuit/min.

^ H28 concentration is reduced to t-he desired level.

The liquor product of the mixing chamber reaction was divided equally, one-half being withdrawn from the system at 19, and one-half being delivered to the top of the sulfitation tower 52 via 23. Each half of the said liquor product had the following composition:

. SOB- 152 moles=17. 84 kg. NazSOa; 1.08 kg. NaHSOa. (12) S2032-. 2.9 moles= 0.46 kg. NazSzOs.

C03-... 3.4 m01es= 0.29 kg. NaHCOa.

SC3-- 152 moles=17.84 kg. NarSOa; 1.08 kg. NaBSOa. (13) S2032..- 2.9 m0leS= 0.46 kg. NiMS/203.

C at.-. 3.4 moles= 0.29 kg. NaHCO3.

The :gas product of the mixing chamber reactions consisting of CO2, inert N2 and traces of O2 and H28, as shown in the gas composition (5) above, was recirculated to the bottom of the carbonation tower 36 via conduit 17.

IV. Sulftation section 'Ille remainder of the gas product (9) of the catalytic conversion having the following composition:

The liquor product of the sultation tower consisting principally of NaHSO3, as shown in liquor composition (10) above, was returned to the mixing chamber 50.

The gas product of the sultation tower 52 having the following composition:

(16) lsqjljgi: ,tasnsmles 768 ouin/min.

was vented to the atmosphere.

The system is suitable to be regulated automatically. The major reasons for readjustment of the operating concentrations are changes in the green liquor ilow and composition. The operation of the absorption tower is deter-mined by the composition of the gases (4) leaving the tower. I-f a CO2 recorder analyzing the gas (4) indicates an increase, the control equipment connected to a damper in the line 32 carrying the gas to the tower reduces the ow until the CO2 concentration and, simultaneously, The operation of the carbonation tower is determined by the temperature of the gas entering the catalyst. If, `for example, the temperature increases, the control equipment connected to a damper in the line 314 increases the ow of the gas circulating through the carbonation tower and the catalyst system. This results in a decrease of the H28 concentration and a lowering of the gas temperature to the desired level. The air ow 40 to the system is regulated by the indications of the oxygen analyzer testing the gas (9) leaving the catalyst, if the gas is converted to SO2. In the case of Example 1, involving the conversion of the H28 to elemental sulfur, the SO2 concentration of this gas is maintained at a certain low level, and the SO2 indicator regulates the addition of air. The amount of `gas that is recirculated through 411's several times the flow of gas entering the catalyst system, and normally no changes of the ilow of gas that is recirculated are necessary. There is a `damper in line 68 that is governed by a slight vacuum indicator 69 hooked up to line l68 before the damper. The SO2 concentration of the gas (16) released to the atmosphere from the bisulfte towerA determines the SO2 flow to the bisulfte tower. As a considerable amount of liquor is contained in the tank the regulation of the SO2 liow, i.e., the sulfur burner, will be slow. The liquor flow to the bisullite tower 52 from the mixing tank 50 and the recycle to the latter is Ydetermined -by the pH of the liquor in the tank 50. If a relatively4 high pH is maintained in the liquor of tank 50ct from 6.5-7.5, the solution contains carbonate and suliite, or suliite with little or no excess bisulfite. The SO2 content of the gas (16) ythat is relieved via 21 from the suliitation tower 52 is low in this case. A lower pH means the presence of an increasing amount of bisuliite. The SO2 concentration of gas (16) relieved from the sultation tower 52 increases unless the tower is operated at a lower temperature. If a signiicant amount of SO2 appears in gas (16), this gas is piped to the duct of the recovery furnace iiue gases through fan 71 before they pass the absorption towers. However, if it is required to produce a liquor containing a relatively high concentration of tree SO2, the liquor leaving the installation is tapped ott at 15 from the suliitation tower, rather than -from tank 50 at 19.

The sample .flowing to the pH recorder is heated to a slightly higher constant temperature before it reaches the electrodes.

The conversion of the green liquor to a sodium carbonate-bicarbonate solution makes it possible to eliminate an accumulation of, for example, sodium chloride. By lowering the temperature of the liquor leaving the carbonation tower in the presence of CO2 (from tank 50),V a substantial part of the sodium can be precipitated as sodium bicarbonate. The liquor is decanted or iiltered oli, and the sodium `bicarbonate is added to the mixing tank 50. The gas from the catalyst system in line 60 is piped directly to line 17. The filtrate is returned to the green liquor smelt dissolving tank, or discarded.

The acid suliite spent liquor can be neutralized, and the sodium-sulfur ratio in the spent liquor can be raised by operating one of the smelt dissolving tanks periodically or continuously with spent liquor. The neutralization not only reduces the corrosiveness of the spent liquor, but increases at the same time-the amount of sulfur that can be processed in the green liquor-sodium sulfite conversion system. The CO2 and HZS developed during the neutralization is added to the 'gas in line 34 that is piped to the catalyst system.

' Having thus described my invention and illustrated it by preferred embodiments, I claim as new and desire to protect by Letters Patent:

1. The method of continuously converting the-sodium suliide and sodium carbonate content of green liquor, derived from a smelt of the waste liquor from the pulpiug of wood, to sodium suliite for re-use in the pulping process, which method comprises a CO2 and HZS absorption stage (A), a carbonation stage (B), an H28 conversion stage (C), a'NaZCOS, NaHCO3 and NaI-1803 mixing stage (D), and a sultation stage (E),

passing a gas containing nitrogen, hydrogen suliide and carbon dioxide countercurrently through a descending ow of the green liquor at a temperature of from 60-90 C. in the said absorption stage (A), whereby the carbon dioxide and hydrogen sulde are separated almost quantitatively from the said gas by reacting with some of the sodium sulde and sodium carbonate of the liquor to produce sodium hydrosulde and sodium bicarbonate;

Y withdrawing the remainder of said -gas comprising almost entirely nitrogen from said stage (A) by maintaining-the system under a slight vacuum;

subjecting the residual liquor from the said absorption stage (A) containing sodium hydrosultide, sodium sulfide, sodium carbonate, 'and sodium bicarbonate and having a relatively constant and higher H28 vapor pressure than the original green liquor in said carbonation stage (B) to the countercurrent action of a gas comprising principally carbon dioxide, some nitrogen and water vapor, said gas having a flow rate suicient to maintain a ratio of carbon dioxide to sullide entering said stage (B) of at least 8 mols to 1, whereby to convert the sodium suliide and sodium hydrosuliide to a non-inflammable gas product containing hydrogen sulfide up to 6% and a liquor product containing sodium bicarbonate and sodium carbonate;

dividing the gas product of said carbonation stage into two portions, one of which is recycled to the said absorption stage (A) to provide the gas reactant of that stage, and the other major portion is passed onto said conversion stage (C) where the hydrogen suliide is mixed with air and'catalytically oxidized at a temperature of from 200- 400 C. to a member selected from the group consisting of elemental sulfur and sulfur dioxide;

passing any elemental sulfur produced to a sulfur conversion stage where it is oxidized to sulfur dioxide;

heating the gas entering the ,said conversion stage (C) with the hot gases leaving said Stage (C) to maintain an even temperature during the conversion;

contacting at least a major portion of the effluent gas from the conversion stage with the liquor product of the carbonation stage (B), and the remainder of the said efuent gas, with the liquor of said sultation stage (E);

reacting sodium bisuliite from the said stage (E) with the liquor product of said stage (B) containing the sodium carbonate and sodium bicarbonate in said mixing stage (D) to produce a gas product containing essentially carbon dioxide and a liquor product containing essentially sodium suliite;

returning the last-mentioned gas product to the carbonation stage (B) to provide-the carbon Vdioxide reactant of that stage;

removing from the system a irst portion 0f the lastrnentioned liquor product containing essentially sodium suliite, said portion being equal to the green liquor flow as produced sodium sulte liquor, and passing a second portion of it in the said suliitation stage (E) countercurrently to a flow of sulfur dioxide gases whereby to produce a gas product consisting essentially of nitrogen and a sodium bisulte liquor product, returning the latter to said mixing stage (D) to provide the sodium Vbisullite reactant of that stage, and relieving the said gas product from said stage (E)` 2. The method of claim 1 in which about a stoichiometric proportion of air is added at said conversion stage (C) to convert the H28 principally to elemental sulfur, the gas product of said conversionrstage (C) is cooled to l20-150 C., and said major portion thereof is combined with the gas product of said mixing stage (D).

3. The method of claim l in which sufficient excess of air isV added at the conversion stage (C) to convert the HZS principally to SO2, and the gas product is divided, a major portion passing to the mixing stage (D) and a smaller portion passing to the suliitation stage (E).

4. The method of claim 1 in which the H28 concentration of the gas produced in the carbonation stage is maintained at a concentration of 1.0-6.0%.

5. The process of claim 1 in which a major portion of the hydrogen sulfide produced in the carbonation stage (B) is mixed with air and catalytically converted to sulfur, the sulfur thus produced is oxidized with make-up sulfur to sulfur dioxide, and the resulting suifur dioxide is reacted with the sodium sulte in the sultitation stage (E).

6. The method of claim 2 in which the gas product from the HZS conversion stage (C) is subjected to a washing stage prior to passing onto the mixing stage (D) to remove carry-over elemental sulfur and react the residual HZS and SO2 to form additional elemental sulfur in the presence of water.

7. The method of claim 6 in which the wash liquor spent pulping liquor and, after the washing stage, 1s added to additional spent pulping liquor for smelting.

8. The method of claim l in-which the ow of gas through the carbonation stage (B) is increased in response to an increase in the temperature of the gas entering the catalytic conversion stage (C); the gas flow through the absorption stage (A) is reduced in response to an increase in the conrellivfill 0f CO2 and HES in the -gas leaving said absorption stage (A); the ow of bisullte liquor to the mixing stage (D) is increased in response to an increase in the pH of the liquor in said mixing stage (D).

9. The method of claim 1 in which the gas entering the conversion stage (C) is heated by recycling a. controlled proportion of said gas leaving the conversion stage with the entering gas.

10. The method `of claim 1 in which the gas entering the conversion stage (C) is heated by a heat exchange with the reaction gas mixture leaving the conversion stage.

11. The method of claim 1 in which the major portion of the eiuent gas from the conversion stage (C) is contacted in the mixing stage with the liquor product of the carbonation stage (B) mixed with the liquor product of the sultation stage (E).

References Cited in the le of this patent UNITED STATES PATENTS OTHER REFERENCES Bone et al.: Flame and Combustion in Gases, Longmans, Green and Co., N.Y., 1927, page 488. 

1. THE METHOD OF CONTINUOUSLY CONVERTING THE SODIUM SULFIDE AND SODIUM CARBONATE CONTENT OF GREEN LIQUOR, DERIVED FROM A SMELT OF THE WASTE LIQUOR FROM THE PULPING OF WOOD, TO SODIUM SULFITE FOR RE-USE IN THE PULPING PROCESS, WHICH METHOD COMPRISES, A CO2 AND H2S ABSORPTION STAGE (A), A CARBONATION STAGE (B), AND H2S CONVERSION STAGE (C), A NA2CO3, NAHSO3 MIXING STAGE (D), AND A SULFITATION SATAGE (E), PASSING A GAS CONTAINING NITROGEN, HYDROGEN SULFIDE AND CARBON DIOXIE COUNTERCURRENTLY THROUGH A DESCENDING FLOW OF THE GREEN LIQUOR AT A TEMPERATURE OF FROM 60*-90* C. IN THE SAID ABSORPTION STAGE (A), WHEREBY THE CARBON DIOXIDE AND HYDROGEN SULFIDE ARE SEPRATED ALMOST QUANTITATIVELY FROM THE SAID GAS BY REACTING WITH SOME OF THE SODIUM SLUFIDE AND SODIUM CARBONATE OF THE LIQUOR TO PRODUCE SODIUM HYDROSULFIDE AND SODIUM BICARBONATE, WITHDRAWING THE REMAINDER OF SAID GAS COMPRISNG ALMOST ENTIRELY NITROGEN FROM SAID STAGE (A) BY MAINTAINING THE SYSTEM UNDER A SLIGHT VACUUM, SUBJECTING THE RESIDUAL LIQUOR FROM THE SAID ABSORPTION STAGE (A) CONTAINING SODIUM HYDROSULFIDE, SODIUM SULFIDE, SODIUM CARBONATE, AND SODIUM BICARBONATE AND HAVING A RELATIVELY CONSTANT AND HIGHER H2S VAPOR PRESSURE THAN THE ORIGINAL GREEN LIQUOR IN SAID CARBONATION STAGE (B) TO THE COUNTERCURRENT ACTION OF A GAS COMPRISING PRINCIPALLY CARBON DIOXIDE, SOME NITROGEN AND WATE VAPOR, SAID GAS HAVING A FLOW RATE SUFFICIENT TO MAINTAIN A RATIO OF CARBON DIOXIDE TO SULFIDE ENTERING SAID STGE (B) OF AT LEAST 8 MOLS TO 1, WHEREBY TO CONVERT THE SODIUM SULFIDE AND SODIUM HYDROSULFIDE TO A NON-INFLAMMABLE GAS PRODUCT CONTAINING HYDROGEN SULFIDE UP TO 6% AND A LIQUOR PRODUCT CONTAINING SODIUM BICARBONATE AND SODIUM CARBONATE, DIVIDING THE GAS PRODUCT OF SAID CARBONATION STAGE INOT TWO PORTIONS, ONE OF WHICH IS RECYCLED TO THE SAID ADSORPTION STAGE (A) TO PROVIDE THE GAS REACTANT OF THAT STAGE, AND THE OTHER MAJOR PROVIDE THE GAS REACTANT OF THAT STAGE, STAGE (C) WHERE THE HYDROGEN SULFIDE IS MIXED WITH AIR AND CATAYLTICALLY OXIDIZED AT A TEMPERATURE OF FROM 200400*C. TO A MEMBER SELECTED FROM THE GROUP CONSISTING OF ELEMENTAL SULFUR AND SULFUR DIOXIDE, PASSING ANY ELEMENTAL SULFUR PRODUCED TO A SULFUR CONVERSION STAGE WHEREIN IT IS OXIDIZED TO SULFUR DIOXIDE, HEATING THE GAS ENTERING THE SAID CONVERSION STAGE (C) WITH THE HOT GASES LEAVING SAID STAGE (C) TO MAINTAIN AN EVEN TEMPERATURE DURING THE CONVERSION, CONTACTING AT LEAST A MAJOR PORTION OF THE EFFLUENT GAS FROM THE CONVERSION STAGE WITH THE LIQUOR PRODUCT OF THE CARBONATION STAGE (B), AND THE REMAINDER OF THE SAID EFFLUENT GAS, WITH THE LIQUOR OF SAID SULFITATION STAGE (E), REACTING SODIUM BISULFITE FROM THE SAID STAGE (E) WITH THE LIQUOR PRODUCT OF SAID STAGE (B) CONTAINING THE SODIUM CARBONATE AND SODIUM BICARBONATE IN SAID MIXING STAGE (D) TO PRODUCE A GAS PRODUCT CONTAINING ESSENTIALLY CARBON DIOXIDE AND A LIQUOR PRODUCT CONTAINING ESSENTIALLY SODIUM SULFITE, RETURNING THE LAST-MENTIONED GAS PRODUCT TO THE CARBONATION STAGE (B) TO PROVIDE THE CARBON DIOXIDE REACTANT OF THAT STAGE, REMOVING FROM THE SYSTEM A FIRST PORTION OF THE LASTMENTIONED LIQUOR PRODUCT CONTAINING ESSENTIALLY SODIUM SULFITE, SAID PORTION BEING EQUAL TO THE GREEN LIQUOR FLOW AS PRODUCED SODIUM SULFITE LIQUOR, AND PASSING A SECOND PORTION OF IT IN THE SAID SULFITATION STAGE (E) COUNTERCURRENTLY TO A FLOW OF SULFUR DIOXIDE GASES WHEREBY TO PRODUCE A GAS PRODUCT CONSISTING ESSENTIALLY OF NITROGEN AND A SODIUM BISULFITE LIQUOR PRODUCT, RETURNING THE LATTER TO SAID MIXING STAGE (D) TO PROVIDE THE SODIUM BISULFITE REACTANT OF THAT STAGE, AND RELIEVING THE SAID GAS PRODUCT FROM SAID STAGE (E). 